In refrigerant or other gas distribution systems employing multiple compressors in parallel for compressing the gas and distributing the same through a common discharge manifold and wherein a common suction manifold feeds the suction side of the multiple compressors from the end use device or devices, there arises problems due to the fact that the gas tends to carry the lubrication oil and the oil may excessively accumulate within one or more of the compressors, while others are simply starved of oil.
Attempts have been made to provide bleed line connections between such compressors and utilize gas pressure differential between the compressors to circulate oil to the starved compressor. Where the compressors constitute hermetic units, the bottom of the casing of the compressor acts as an oil sump for receiving accumulated oil and for supplying oil to the moving parts of the compressor unit for cooling, lubrication and sealing. One such oil distribution arrangement is the subject matter of U.S. Pat. No. 3,237,852 assigned to the common corporate assignee and applied to a compressed gas distribution system employing multiple parallel hermetic motor compressor units of the reciprocating piston type.
Additionally, oil equalization schemes have been employed for parallel connected hermetic helical screw compressor units. One such oil equilization system is the subject of U.S. Pat. No. 4,179,248, also assigned to the common corporate assignee. Such helical screw compressor is formed of intermeshed rotary helical screws which are vertically oriented and mounted for rotation about parallel, vertical axes within a hermetic compressor casing. An electric drive motor for the helical screw rotors is vertically mounted above one of the helical screws and shaft connected thereto. Further, to assist in oil distribution and oil separation, the interior of the hermetic casing is maintained at compressor discharge pressure with the major portion of the separated oil accumulating within the bottom of the casing and rising to a level less than the vertical height of the intermeshed screws and well below the level of the electric motor. In the oil equalization system of the latter patent, the multiple helical screw rotary compressor units are connected in parallel across common suction and discharge manifolds. The discharge manifold opens to each unit casing well above the level of accumulated oil within the bottom of the hermetic casings acting as oil sumps. Oil bleed lines are provided for the compressors which open to the casing interior at an oil bleed port at the normal level of accumulated oil. The bleed lines are of small diameter and open at their opposite ends directly to the discharge manifold at points downstream of the connection between the discharge manifold and the hermetic casings for the screw compressors. Excessive accumulation of oil within one of the hermetic compressor casings causes the oil to bleed through its bleed line under a small pressure differential to the discharge manifold for redistribution to the remaining compressors through the system proper, that is, by being carried along with the compressed working fluid.
While such oil equilization system works adequately in terms of insuring that a compressor having an excess amount of oil feeds those compressors being starved of oil, the system is fraught with problems since of necessity, some of the oil is circulated with the working fluid in order to achieve redistribution of the oil from other hermetic compressor to the compressor which is starved of oil. Also, this requires a large quantity of oil to be in circulation in system which is not available to lubricate, etc.
It is, therefore, a primary object of the present invention to provide an improved oil equalization scheme for a multiple compressor gas distribution system employing parallel connected compressor units or the like in which, automatically, accumulated oil is bled from a unit having an ample reserve and circulated to the remaining compressors or compressor units which are starved of oil only when compressor(s) is/are in need of oil and in which such circulation is achieved without the necessity of using the compressed gas as the distribution means with no effect on refrigeration system or heat transfer components.